That may be a good percentage, or, it indicates they only prosecute the worst of the worst cases and dismiss the rest.
There is an additional reason I don't see mentioned. I have a friend that actually went through this process during the Clinton Administration.
He said they threatened to bankrupt him and put his family out on the street if he didn't plead guilty. He said they told him that if he beats the initial charge, they will file a new charge against him, and they will grind him into the dust financially. They will deny him his Federal pension (he was a federal employee) and still put him in jail.
They said that if he would just agree to plead guilty, he could keep his pension and his wife and kids could keep their house. He would serve a few years and then be released.
He plead guilty.
For more background, he claims his boss did what he was accused of doing, and he proved it to the federal prosecutors. They said they didn't care. His boss was black, and they weren't going to prosecute him, but they had to prosecute somebody, so he was it.
It’s the latter, they personally go after people who are guilty as sin, and highly incriminating, most of the time, the other part is that many of these criminals who travel around the nation over multiple states such as serial killers and repeat con artists tend to get comfy with what they do and mess up along the way in terms of covering up their tracks. Or they feel the need to boast about their acts to someone who calls them in.